<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title>练习</title>
</head>
<body>
    <input type="text" id="calculate" />
    <button id="btn"></button>
</body>
<script>
    var btn=document.getElementById("btn");
    var calculate=document.getElementById("calculate")
    //当点击按钮时，将文本框中运算式运算结果alert出来
    btn.onclick=function(){
        var result=eval(calculate.value);
        result=parseInt(result);
        console.log(result);
        alert(calculate.value+"="+result);
        }
</script>
    
</html>